The present invention relates to fully tunable photonic band gap (PBG) materials and recipes for synthesis of these materials using periodic composite dielectric materials having differing indexes of refraction in which one or both of the indexes are electro-optically or magneto-optically tunable thereby permitting the photonic band structure to be tuned. More particularly the invention relates to photonic crystals and photonic band gap (PBG) materials having optical stop gaps or complete photonic band gaps in which this stop gap or complete photonic band gap can be opened, closed or readjusted, either locally or globally by means of an external electric, magnetic, or electromagnetic field.
Photonics is the science of molding the flow of light. Photonic band gap (PBG) materials, as disclosed in S. John, Phys. Rev. Lett. 58, 2486 (1987), and E. Yablonovitch, Phys. Rev. Lett. 58, 2059 (1987), are a new class of dielectrics which carry the concept of molding the flow of light to its ultimate level, namely by facilitating the coherent localization of light, see S. John, Phys. Rev. Lett. 53, 2169 (1984), P. W. Anderson, Phil. Mag. B 52, 505 (1985), S. John, Physics Today 44, no. 5, 32 (1991), and D. Wiersma, D. Bartolini, A. Lagendijk and R. Righini, Nature 390, 671 (1997). This provides a mechanism for the control and inhibition of spontaneous emission of light from atoms and molecules forming the active region of the PBG materials, and offers a basis for low threshold micro-lasers and novel nonlinear optical phenomena. Light localization within a PBG facilitates the realization of high quality factor micro-cavity devices and the integration of such devices through a network of microscopic wave-guide channels (see J. D. Joannopoulos, P. R. Villeneuve and S. Fan, Nature 386, 143 (1998)) within a single all-optical microchip. Since light is caged within the dielectric microstructure, it cannot scatter into unwanted modes of free propagation and is forced to flow along engineered defect channels between the desired circuit elements.
The utility of PBG materials arises essentially from their ability to facilitate the localization of light and the controllable inhibition of spontaneous emission of light from atoms and molecules as mentioned above. Although an intensive effort has developed over the past ten years, to microfabricate PBG materials (see xe2x80x9cPhotonic Band Gap Materialsxe2x80x9d edited by C. M. Soukoulis, Kluwer Academic Publishing, Dordrecht 1996; Journal of Lightwave Technology IEEE, volume 17, number 11 (1999) Special Issue on Photonic Crystals), it is only recently that a clear route to synthesizing large scale three-dimensional PBG materials with submicron lattice constants has been demonstrated using self-assembly methods, see K. Busch and S. John, Physical Review E58, 3896 (1998); J. E. G. J. Wijnhoven and W. L. Vos, Science 281, 802 (1998); A. A. Zakhidov et. al. Science 282, 897 (1998). The application of this approach to Si, Ge and GaAs based PBG materials may open the door to applications in laser devices and telecommunications as well as to the realization of fundamentally new effects in quantum and nonlinear optics. As pointed out by Sir John Maddox, xe2x80x9cIf only it were possible to make dielectric materials in which electromagnetic waves cannot propagate at certain frequencies, all kinds of almost magical things would be possible.xe2x80x9d John Maddox, Nature 348, 481 (1990).
Recently, it has been shown that PBG materials based on silicon and germanium can be readily fabricated based on inverse opal structures. The value of the photonic band gap depends on the physical dimensions of the air voids so that fabricating the precursor opal with a given sphere size fixes the photonic band gap, U.S. Pat. No. 60/178,773 filed Jan. 28, 2000 and U.S. Pat. No. 60,202,115 filed May 5, 2000, both references being incorporated herein by reference.
For many applications, however, it would be advantageous to obtain some degree of tunability of the photonic band structure through electro-optic or magneto-optic effects. Such devices would find very broad applications in optical networking for telecommunications. The tunable PBG material would act as a tunable mirror or a switch with a latch for routing optical data from one channel to another. If such tunable PBG materials could be made they would form the basic building blocks in photonic circuits, analogous to the role of semiconductors in conventional microelectronics. In the case of semiconductors, the flow of electricity is regulated by means of applied voltages in most applications (but also by magnetic fields in some applications) which modulates the electronic band structure.
It has been proposed that such tunability may be obtained by controlling one or several forms of optical anisotropy of the constituent materials. The science of liquid crystals (see P. G. de Gennes and J. Prost, xe2x80x9cThe Physics of Liquid Crystalsxe2x80x9d, Clarendon Press, Oxford 1993; S. Chandrasekhar, xe2x80x9cLiquid Crystalsxe2x80x9d, Cambridge University Press 1992; L. M. Blinov and V. G. Chigrinov, xe2x80x9cElectro-Optic Effects in Liquid Crystal Materialsxe2x80x9d, Springer, N.Y. 1994) has spawned an entire industry related to these electro-optic effects. In earlier works, however, a rather pessimistic conclusion regarding the efficacy of birefringent photonic crystals was drawn (see I. H. Zabel and D. Stroud, Phys. Rev. B48, 5004 (1993); Z. Y. Li, J. Wang, B. Y. Gu, Phys. Rev. B58, 3721 (1998); R. M. Hornreich, S. Shtrikman and C. Sommers, Phys. Rev. E47, 2067 (1993)) since attention was restricted to unrealizable structures that consist of spheres of disconnected anisotropic, high dielectric materials in an air background.
It would be very advantageous to provide an economical and technologically simple recipe for combining the high sensitivity of the photonic band structure to small variations in the refractive index with the known refractive index tunability of liquid crystals or ferro-electrics to produce PBG materials which are tunable.
It is an object of the present invention to provide photonic band gap (PBG) materials in which the band structure of a photonic crystal can be changed in a controlled manner.
The present invention provides fully tunable photonic band gap (PBG) materials and recipes for synthesis of these materials using periodic composite dielectric materials having differing indexes of refraction whereby changing the refractive index properties of one or more of the dielectric constituents by application of an external field tunes or modulates the photonic band structure in a predictable way. These composite dielectric materials may comprise a high refractive index dielectric material and another optically anisotropic, birefringent, electro-optically tunable, or magneto-optically tunable material with a lower dielectric constant in which the photonic band structure can be globally or locally changed in a controlled manner by application of an external electric, magnetic, or electromagnetic field.
Therefore, in one aspect of the invention there is provided a photonic crystal having a tunable photonic band structure, comprising;
a periodic composite dielectric material having at least two dielectric constituents including a first dielectric constituent having a first refractive index and a second dielectric constituent having a refractive index smaller than the first refractive index so that the periodic composite dielectric material has a photonic band structure; and
at least one of said at least two dielectric constituents having refractive index properties which can be locally or globally changed throughout said photonic crystal in a controlled manner whereby changing the refractive index properties modulates said photonic band structure locally or globally throughout said photonic crystal for providing control of propagation of light through said photonic crystal.
In another aspect of the invention there is provided a photonic crystal having a tunable photonic band structure, comprising;
a periodic composite dielectric material having a first dielectric constituent having a first refractive index and void regions located periodically throughout a volume of said periodic composite dielectric material, a second dielectric constituent located in said void regions having a second refractive index sufficiently smaller than the first refractive index so that the periodic composite dielectric material has a photonic band structure; and
at least one of said first and second dielectric constituents being optically anisotropic and having refractive index properties which can be locally or globally modified in a controlled manner whereby changing the refractive index properties changes said photonic band structure for providing control of propagation of light through said photonic crystal.
The high refractive index constituent may be a semiconductor material such as silicon, germanium, gallium phosphide, gallium arsenide, indium phosphide or some other high refractive index semiconductor and the photonic band gap (PBG) materials may have optical stop gaps or complete photonic band gaps in which this stop gap or complete photonic band gap can be opened, closed or readjusted, either locally or globally by means of an external electric, magnetic, or electromagnetic field.
In one aspect of the invention the fully tunable PBG material includes an inverted opal comprising periodic air inclusions in a high refractive index semiconductor backbone. An optically anisotropic or birefringent material, and more specifically a low index nematic liquid crystal (or ferro-electric) is infiltrated into the void regions of an inverse opal. The low index nematic liquid crystal may be bis ethylhexyladipate (BEHA), or more generally any material with refractive index anisotropy na=1.6 and nb=1.4. The three-dimensional PBG can be completely opened or closed by applying an electric field which rotates the axis of the nematic molecules (or ferro-electric polarization) relative to the inverse opal backbone. This complete tunability arises from the very high sensitivity of the photonic band structure of the inverse opal structure to small variations in the refractive index. The refractive index change is induced by an external voltage so that the flow of light is steered by an external voltage that modulates the photonic band structure through a linear electro-optic effect.
The present invention is not restricted to using optically anisotropic materials infiltrated into the voids of the backbone. For example, the photonic crystal dielectric composite may be comprised of a tunable backbone, with high refractive index, and a periodic array of air holes. Alternatively, the photonic crystal may comprise a non-tunable backbone and one or more tunable (optically anisotropic) materials which either fill or partially fill the air pores.
In another aspect of the invention there is provided a method of tuning a photonic band structure in a photonic crystal, comprising;
providing a photonic crystal having a periodic composite dielectric material including a first dielectric constituent having a first refractive index, and at least a second dielectric constituent having a second refractive index sufficiently smaller than the first refractive index so that the periodic composite dielectric material has a photonic band structure; and
globally or locally changing the refractive index properties of one of said first and second dielectric constituents in a controlled manner so that said photonic band structure is changed in a controlled manner by application of one of an electric, magnetic and electromagnetic field for providing control of propagation of light through said photonic crystal.